Recent advances in the control of thin films and surfaces have brought an intriguing question within reach: Is it possible to tailor the electronic properties of solids by controlling them layer by layer or row by row? Customized molecules are commonplace in biochemistry. Can the same idea be brought to bear on solids and electronic materials? Electronic properties of semiconductor devices have been controlled by hetero-structures, quantum wells, and super-lattices. Magnetism as a cooperative phenomenon lends itself to manipulation in small structures, where neighbor atoms can be replaced systematically by species with stronger or weaker magnetism. In fact, a class of magnetic/nonmagnetic multilayers termed spin valves has recently been introduced into commercial read heads for magnetically stored data. The optimum thickness of their active region lies in the single-digit-nanometer regime.
The smallest nanostructures may be viewed as objects consisting only of interfaces with no bulk behind them. More typically, single-digit-nanometer dimensions are sufficient for realizing the benefits of structuring (e.g., operating a quantum-well device at room temperature). This regime is difficult to reach with lithography methods, particularly when macroscopic amounts are to be fabricated. Self-assembly becomes the method of choice.